1. Field of the Invention (Technical Field)
The present invention relates generally to devices for the measurement of airspeed and fluid flow rates. Specifically, the invention is of a non-intrusive velocity indicator intended for surfaces over which a boundary layer undergoes transition to turbulence, either naturally or through input disturbances.
2. Background Art
Accurate airspeed measurements are essential to safe operations of jet aircraft. Typically, airspeed is inferred from a differential pressure measurement made with a pitot-static tube. Critical to the proper usage of a pitot-static tube is an estimate of the density of air in the vicinity of the tube. As such, a pitot-static tube must be calibrated for altitude (or corrected by the pilot in the case of general aviation aircraft) as well as Mach number in high-speed flight. The present invention concerns a device for which no density corrections need be applied.
Other prior art devices have been proposed for airspeed measurements. Many rely on various embodiments of a classical thermal anemometry system. These include those described in U.S. Pat. Nos. 4,920,793, 3,995,481, 5,357,795, and 5,639,964, to Djorup, U.S. Pat. No. 4,637,253, to Sekimura et al., U.S. Pat. No. 4,070,908, to Newell, U.S. Pat. No. 5,677,484, to Stark, and U.S. Pat. No. 5,311,775, to Suski et al. Such devices rely on the physical principle that the amount of heat dissipated from a heated element exposed to a moving stream is some function of the freestream speed. U.S. Pat. No. 5,357,795, to Djorup, also employs a vortex-shedding flowmeter in addition to a thermal anemometry system. Such a flowmeter relies on another well-known physical mechanism, namely that over some range of incident flow speeds, a circular cylinder will shed vortices at a frequency which is a unique function of the incident flow speed. However, Djorup appears to use vortex shedding as a simple indicator of flow direction. That is, the sensors which detect vortex shedding frequency must be located in the actual flowstream, while those at other circumferential locations around the axis of the device may or may not be. U.S. Pat. No. 5,639,964, to Djorup, discloses a pair of sensors offset along the major axis of an aircraft and speaks of crosscorrelating the signals from the sensors in order to estimate the likelihood of the aircraft experiencing large scale (i.e. aircraft sized turbulence).
U.S. Pat. No. 5,520,047, to Takahashi, is based on the principle that the heat-flux distribution along a flat plane incident to a moving stream is an indicator of air speed.
While the prior does employ a global measurement of the heat flux distribution over a surface as an indicator of flow speed, the present invention does so by making use of a measurement of the convection speed of turbulent spots as an indicator of flow speed. The present invention is unique in that it directly measures the velocity of events occurring in the flow and uses a simple calibration to infer the true airspeed.